Electronically and Sterically Varied Vanadium Aminopyridinate Complexes Applied to the Catalytic Reductive Coupling of Alcohols

2-Aminopyridinate complexes of vanadium were synthesized,
isolated,
and characterized. Comparative reactivity showed that 2-aminopyridinate
ligands can be used in the synthesis of monometallic complexes and
DFT calculations indicate increased electron donation for the 2-aminopyridinate
ligand on vanadium compared to pyridonate ligands. A homoleptic, monometallic
tris(N-phenyl-2-aminopyridinate)vanadium(III) complex
was used for the catalytic reductive coupling of benzhydrol, and a
series of various 2-aminopyridinate ligands were also screened for
this transformation. Unsubstituted 2-aminopyridine as the proligand
resulted in the highest activity, with the in situ catalytic system demonstrating improved reactivity with benzylic
and allylic alcohols compared to previously reported systems. The
isolation of the vanadium aminopyridinate complexes, coupled with
kinetic investigations, supports monometallic intermediates for the
catalytic reductive coupling of alcohols.